Swift学习三：闭包Closures

定义

闭包（Closures）是独立的函数代码块，能在代码中传递及使用。

语法

{(parameters) -> return type in 　　statements }

注：闭包表达式语法可以使用常量参数、变量参数和 inout 类型作为参数，但皆不可提供默认值。

范例

func funA(var value : String,closure: (str : String) -> ()) { closure(str:value) } func closureA(var str : String){ println("closureA : Hi," + str) } func closureB(var str : String){ println("closureB : Hi," + str) } funA("cosmokey",closureA)    //closureA : Hi,cosmokey
funA("cosmokey",closureB)    //closureB : Hi,{(var str : String) in println("closureC : Hi," + str)})    //closureC : Hi,cosmokey

语法糖

根据上下文推断类型 funA("cosmokey",{str in println("closureD : Hi," + str)})    //closureD : Hi,cosmokey
 参数名简写 funA("cosmokey",{println("closureE : Hi," + $0)})    //closureE : Hi,cosmokey
 运算符函数 sort(names,>)  //sort(names,{ (s1: String,s2: String) -> Bool in return s1 > s2 } ) 

Trailing 闭包

注：Trailing 闭包是一个书写在函数括号之外(之后)的闭包表达式，函数支持将其作为最后一个参数调用。 funA("cosmokey"){ str in println("closureF : Hi," + str)     //closureF : Hi,cosmokey
}

系统常见闭包：map、filter和reduce

map map 可以把一个数组按照一定的规则转换成另一个数组 定义 func map<U>(transform: (T) -> U) -> U[] 范例 var oldArray = ["Ping","Cosmokey"] var newArray = oldArray.map(){"Hi," + $0}    //["Hi,Ping","Hi,Cosmokey"]

filter filter 起到的就是筛选的功能，参数是一个用来判断是否筛除的筛选闭包 定义 func filter(includeElement: (T) -> Bool) -> [T] 范例 var oldArray = ["Ping","Cosmokey","MM"] var newArray = oldArray.filter(){count($0) > 2}  //["Ping","Cosmokey"]

reduce reduce 函数解决了把数组中的值整合到某个独立对象的问题。 定义 func reduce<U>(initial: U,combine: (U,T) -> U) -> U 范例 var oldArray = ["C","o","s","m","k","e","y"] var newArray = oldArray.reduce("Hi,"){ var newValue = $0 + $1 println(newValue) return newValue } /* Hi,C Hi,Co Hi,Cos Hi,Cosm Hi,Cosmo Hi,Cosmok Hi,Cosmoke Hi,Cosmokey */ println(newArray) //Hi,Cosmokey